1. Field of Endeavor
The present invention relates to a combustion device for a gas turbine. In particular the invention relates to a second combustion device of a sequential combustion gas turbine; sequential combustion gas turbines are known to have two rows of combustion devices, a second row being fed with the flue gases (still containing oxygen) coming from a first row of combustion devices.
The present invention may also be implemented in different combustion devices, such as in combustion devices of the first combustion device row of a sequential combustion gas turbine or in a traditional gas turbine having one single row of combustion devices.
For sake of clarity, simplicity and brevity in the following, specific reference to a combustion device of a second combustion device row of a gas turbine will be made.
2. Brief Description of the Related Art
During operation of gas turbines, heavy thermo-acoustical pulsations may be generated; these pulsations are very detrimental for the gas turbine lifetime (they can cause mechanical and thermal damages) and may also limit the operating regime; thus thermo-acoustical pulsations must be suppressed.
In particular, gas turbines operating with lean premixed, low emission combustion devices exhibit a high risk of unstable combustion that may cause these thermo-acoustical pulsations.
Traditionally, in order to suppress thermo-acoustical pulsations, damping devices connected to the combustion device are provided; examples of such damping devices are quarter wave tubes, Helmholtz dampers, or acoustic screens.
U.S. Patent Application Pub. No. 2005/0229581 discloses a combustion device having an inner and an outer perforated, spaced apart, parallel walls, with the volume between these walls that defines a plurality of Helmholtz dampers (thanks to the holes in the inner wall).
Cooling is a major problem in this structure and is achieved by impingement cooling, by air that, passing through the perforated outer wall, impinges on the perforated inner wall, to then enter the combustion device via the perforated inner wall.
U.S. Pat. No. 6,351,947 discloses a similar combustion device having an additional noise absorbing perforated plate between the spaced apart inner and outer wall, to increase damping effectiveness and frequency bandwidth.
Nevertheless, these combustion devices have a number of drawbacks.
In fact, in order to cool the outer and the inner wall (that delimits the inside of the combustion device), a large amount of air must be diverted through the holes of the outer wall into the space between the inner and outer wall.
This reduces the damping efficiency and, since this air does not take part in the combustion, the flame temperature and consequently the NOx emissions are higher than what is theoretically possible.
This drawback is even greater in the combustion devices having the noise absorbing perforated plate between the inner and the outer wall, since air (that is supplied via holes in the outer wall) cannot directly reach and impinge on the inner wall.
In addition, poor cooling may cause the temperature inside of the space between the inner and outer wall to rise, leading to an increase of the speed of the sound and thus shifting the damping frequency to a frequency different from the design frequency.